The Elastic Spiral Phase Pipe

TL;DR

This paper introduces the elastic spiral phase pipe, a passive device that converts axisymmetric ultrasonic waves into non-axisymmetric flexural waves in pipes, using a design inspired by optical spiral phase plates, supported by theory, simulations, and experiments.

Contribution

It presents the first elastic analogue of optical spiral phase plates for guided elastic waves, enabling efficient mode conversion in pipes with a novel design and spectral collocation method.

Findings

Successful design and implementation of the elastic spiral phase pipe.

Numerical simulations confirm the mode conversion efficiency.

Experimental verification demonstrates practical applicability.

Abstract

We design a device for the passive mode conversion of guided, axisymmetric, ultrasonic waves in hollow elastic pipes into arbitrary non-axisymmetric flexural waves that have a constant angular profile along the pipe axis. To achieve this we create an elastic analogue to optical spiral phase plates - the elastic spiral phase pipe. Three possible configurations of the elastic spiral phase pipe are presented which allow the efficient generation of non-axisymmetric flexural waves from an axisymmetric, longitudinal forcing. The theory leverages the dispersive nature of the guided elastic waves that are supported in pipes through a defined relative refractive index. As such we include a spectral collocation method used to aid the design of the elastic spiral phase pipe that is corroborated with numerical simulations and then experimentally verified.